Image intensifiers and method of producing same

ABSTRACT

A shielded faceplate for use in an image intensifier having a photocathode which comprises an inner core of glass transparent to light to which the photocathode is sensitive and an outer shielding region of glass which absorbs light to which the photocathode is sensitive. The outer shielding region surrounds and is adjacent to the inner core. The inner core has a transverse dimension which decreases along its length, the transverse dimension being smallest at the photocathode surface. The shielded faceplate is manufactured by arranging in a mold to which glass does not bond a first type of glass transparent to light to which the photocathode is sensitive and a second type of glass which absorbs light to which the photocathode is sensitive, heating the glass members so that they fuse to produce a block having an inner core of the first type of glass and an outer region of the second type of glass and removing the block from the mold.

FIELD OF THE INVENTION

This invention relates to image intensifiers and is particularlyconcerned with the image intensifier faceplates and a method ofmanufacturing them.

BACKGROUND OF THE INVENTION

A conventional image intensifier is illustrated in FIG. 1, which is alongitudinal section. It comprises a glass faceplate 1 on which aphotocathode 2 is laid down, a micro-channel plate electron multiplier 3and a phosphor screen 4 on a glass substrate 5. The glass faceplate 1and substrate 5 form the end windows, of a vacuum envelope in which theother elements are contained. In operation an optical image is focussedon the photocathode 2 by an external lens, causing electrons to beemitted. The electrons are accelerated to the electron multiplier 3where they are increased in number by secondary emission at the channelwalls. The secondary electrons are then accelerated towards the phsophorscreen 4 by a potential difference set up between it and the electronmultiplier 3, producing an optical image corresponding to the imagefocussed on the photocathode 2 but of very much greater intensity. Byincreasing the potential difference, a typical value being severalthousand volts, the intensification produced is increased. The flatsurfaces of the photcathode 2, electron multiplier 3, and phosphorscreen 4 must be closely spaced and parallel to ensure good definitionin the final image. To achieve this without discharge breakdownoccurring between electrical connections 6 to the photocathode 2,electron multiplier 3 and phosphor screen 4, the glass faceplate 1 andglass substrate 5 have central raised portions extending towards theelectron multiplier 3. Thus the electrical connections 6 are spaced muchfurther apart than would otherwise be the case.

In such an image intensifier light incident at the periphery of theglass faceplate 1 may undergo reflection within the faceplate 1, asshown at 7, resulting in spurious signals. This may be reduced byemploying a shielded faceplate consisting of a central transparentregion surrounded by light absorbing material.

In a previous method of manufacturing such a shielded faceplate acylindrical clear glass rod 8 is inserted into a black glass tube 9, asshown in FIG. 2. The rod 8 and tube 9 are surrounded by a tubular clearglass envelope 10 which is evacuated. The temperature is then raiseduntil the glasses fuse (FIG. 3) producing a solid cylindrical block.This is sliced transversely to its longitudinal axis X. One of theslices is shown in FIG. 4. The slice is then machined to the requiredshape shown in FIG. 5, to produce a shielded faceplate having a clearglass core 11 surrounded by an outer region of black glass 12.

This method has a number of disadvantages. The machining required isexpensive and, in the case of obtaining the initial black glass tube, isextremely difficult, since its inner surface must be polished to a highquality. In addition, distortions are introduced during the heatingprocess, resulting in a loss of concentricity. Also the parallel sidesof the clear core 11 cause vignetting, or fading of light entering thefaceplate at the periphery of the core 11.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof manufacturing a shielded faceplate for use in an image intensifiercomprising a photocathode, including the steps of: arranging in a mold amember of a first type of glass, transparent to light to which thephotocathode is sensitive, adjacent another member of a second type ofglass which absorbs light to which the photocathode is sensitive, themold being of a material to which glass does not fuse; heating themembers so that they fuse, thereby forming a block having a shapeconforming to the internal configuration of the mold and having an innerregion of the first type of glass and an outer region of the second typeof glass, and removing the block from the mold. The term "light" in thisSpecification should be taken to include infrared and ultra-violetradiation in addition to visible light.

If the photocathode is, for example, sensitive only to near infra-redradiation and the red end of the visible spectrum, the second type ofglass could be blue glass. This is advantageous because it may then bevisually inspected for flaws, unlike black glass. Since the glasses areshaped by the mold the difficulty and expense of machining incurred inemploying the previous method described above is avoided. Also themethod according to the invention is much more flexible than theprevious method, allowing different shielding configurations to beeasily manufactured, and the distortions which occur with the previousmethod are reduced.

It is preferred that the block formed by the method includes a portionof the first type of glass which is completely encircled by a portion ofthe second type of glass.

Preferably, the first and second types of glass have substantially thesame refractive index. This reduces reflections at the boundary betweenthe two and hence the possibility of spurious signals being generated.

It is preferred that the members are discs which have flat surfacesarranged adjacent each other in the mold. Preferably some of the secondtype of glass is removed to expose a flat surface of the first type ofglass which is bounded by the encircling portion of the second type ofglass, allowing light to pass from one side to the other of thefaceplate, although the disc of the second type of glass may be annular,in which case such further processing may not be necessary.

Preferably, the members are heated in an inert atmosphere, preventingoxidation.

According to a feature of the invention, a shielded image intensifierfaceplate is manufactured by a method according to the invention, andpreferably the outer region surrounds the inner region which has atransverse dimension which decreases along its length, being smallest atthe photocathode surface.

According to a second aspect of this invention there is provided ashielded faceplate for an image intensifier including a.[.phtocathode,.]. .Iadd.photocathode, .Iaddend.comprising an inner coreof glass transparent to light to which the photocathode is sensitive andan outer shielding region of glass which absorbs light to which thephotocathode is sensitive which surrounds and is adjacent to the innercore, the inner core having a transverse dimension which decreases alongits length, being smallest at the photocathode surface. Such aconstruction does not tend to suffer from vignetting to the same extentas a conventional shielded faceplate having a parallel sided inner core.Even where the refractive indices of the glasses forming the inner coreand outer shielding differ by only a small amount, incoming lightstriking the boundary between them at a glancing angle (i.e. at a largeangle of incidence) will result in a reflected beam of large amplitude.This condition is less likely to occur when the diameter of the innercore .[.inceases.]. .Iadd.increases .Iaddend.from its smallest value atthe photocathode to the front of the faceplate which receives the light.

Thus the shielded faceplate permits a much improved operation of animage intensifier; and according to a third aspect of this invention, animage intensifier includes an electron multiplier; a phosphor screenupon which electrons from said multiplier are incident; a photocathodeadapted to receive illumination and to emit electrons to the electronmultiplier; and a glass faceplate which supports the photocathode, theglass faceplate being in accordance with this invention as previouslyset out.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional image intensifier;

FIG. 2 illustrates a step in the manufacture of the image intensifier ofFIG. 1;

FIG. 3 shows a solid cylindrical block obtained from a second step inthe manufacture of the image intensifer of FIG. 1;

FIG. 4 is a transverse slice taken through the solid cylindrical blockof FIG. 3;

FIG. 5 shows the transverse slice of FIG. 3 after machining;

FIG. 6 shows the initial glass members;

FIGS. 7 to 11 illustrate a method in accordance with the invention andare transverse sections; and

FIG. 12 illustrates an image intensifier in accordance with theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 6 a clear glass disc 13 having a diameter of 30mm and 9 mm thickness and a black glass disc 14 with a diameter of 30 mmand 1.2 mm thick are polished on all surfaces and comprise the startcomponents. They are placed in a cavity of a graphite mold 15 having amovable member 15A (FIG. 7) with the clear glass disc 13 above the blackglass disc 14 as shown. The temperature is then raised to about 800° C.,when the glasses soften (FIG. 8) and the member 15A moved inwards sothat they are forced into contact with the inner walls of the mold 15defining the cavity. The glasses fuse to form a block 16 having a clearglass region 17 and a region 18 of black glass, as shown in FIGS. 9 and10. The heating process takes place in an inert atmosphere to preventoxidation of the mold 15, and initially at reduced pressure.

Fusion will commence in a circumferential region, and thus any gasevolved from the glass surface during the fusion will form a bubble orblister in the enclosed central region. Since at a later stage this partof the block 16 is usually removed (to permit light to pass through theblock 16 to the photocathode) the bubble or blister may not beobjectionable. However if there are an undesirable number of bubbles theproblem may be reduced by drilling a small hole in the centre of theblack glass disc 14 prior to the heating process. Then any gases evolvedduring fusion may escape. The block 16 is then removed from the mold.

The plane surfaces of the block 16 are then ground down to size (FIG.11), removing a layer of black glass 19 to form a flat surface 20 (towhich the photocathode can be applied), and the surfaces polished. Thefaceplate thus formed has an inner cylindrical region 21, the radius ofwhich increases along its length from the photocathode surface 20 to thefront surface 22 of the faceplate, and a surrounding shielding region 23of black glass. That is, the inner cylindrical region is in the shape ofa truncated cone wherein the upper front surface 22 has a large diameterthan the lower flat surface 20.

The faceplate manufactured as described above with reference to FIGS. 6to 11 may then be included in an image intensifier as shown in FIG. 12.

We claim:
 1. A method of manufacturing a shielded faceplate for use inan image intensifier comprising a photocathode, including the steps of:arranging in a mold a member of a first type of glass transparent tolight to which the photocathode is sensitive, adjacent another member ofa second type of glass which absorbs light to which the photocathode issensitive, the mold being of material to which glass does not bond;heating the members so that they fuse, thereby forming a block having ashape conforming to the internal configuration of the mold and having aninner region of the first type of glass and an outer region of thesecond type of glass; and removing the block from the mold.
 2. A methodas claimed in claim 1 and wherein the block so formed includes a portionof the first type of glass which is completely encircled by a portion ofthe second type of glass.
 3. A method as claimed in claim 1 and whereinthe first and second types of glass have substantially the samerefractive index.
 4. A method as claimed in claim 1 and wherein themembers are discs which have flat surfaces arranged adjacent each otherin the mold.
 5. A method as claimed in claim 4 and wherein, after theblock is formed, some of the second type of glass is removed to expose aflat surface of the first type of glass which is bounded by theencircling portion of the second type of glass.
 6. A method as claimedin claim 1 and wherein the members are heated in an inert atmosphere. 7.A method as claimed in claim 1 and wherein the members are initiallyheated at reduced pressure.
 8. A method as claimed in claim 1 andwherein the material of the mold is graphite.
 9. A shielded faceplatefor an image intensifer including a photocathode, comprising;an innerglass member having the shape of a truncated cone, the upper surface ofsaid inner glass member having a greater diameter than the lower surfacethereof, said inner glass member being transparent to light to whichsaid photocathode is sensitive; and an outer shielding glass membersurrounding and contiguous with the outer surface of said inner glassmember, said outer shielding glass member absorbing light to which saidphotocathode is sensitive, said photocathode being deposited on thelower surface of said inner glass member.
 10. A .Iadd.method ofmanufacturing a .Iaddend.shielded image intensifier faceplate as claimedin claim 1 and wherein the inner region has a transverse dimension whichdecreases along its length, being smallest at the photocathode surface,and is surrounded by the outer region.
 11. A shielded faceplate for animage intensifier including a .[.phtocathode.]..Iadd.photocathode.Iaddend., comprising an inner core .Iadd.member.Iaddend.of glass transparent to light to which the photocathode issensitive, and an outer shielding .[.region.]. .Iadd.member .Iaddend.ofglass which absorbs light to which the photocathode is sensitive whichsurrounds and is .[.adjacent.]. .Iadd.fused .Iaddend.to the inner core.Iadd.member, there being a distinct boundary between said inner coremember and said outer shielding member.Iaddend., the inner core.Iadd.member .Iaddend.having a transverse dimension which decreasesalong its length, being smallest at the photocathode surface.
 12. Animage intensifier including an electron multiplier; a phosphor screenupon which electrons from said multiplier are incident; a photocathodeadapted to receive illumination and to emit electrons to the electronmultiplier; and a glass faceplate which supports the photocathode, thefaceplate being as claimed in claim
 11. 13. A method of manufacturing ashielded faceplate for use in an image intensifier having aphotocathode, said method including the steps of:providing a mold havingan inner wall defining a cavity which symmetrically surrounds a verticalaxis of said mold, the diameter of said cavity being greater at theupper end of said mold than at the lower end thereof, said mold beingformed of a material to which glass does not bond; placing an opaqueglass disc into said cavity, said opaque glass disc being made of amaterial which absorbs light to which said photocathode is sensitive;placing a transparent glass disc into said cavity with its lower surfaceadjacent the upper surface of said opaque glass disc, said transparentglass disc being made of a material which is transparent to light towhich said photocathode is sensitive; heating said glass discs to atemperature at which said discs become soft; forcing said glass discsinto contact with the inner walls of said mold, said glass discs therebyforming a block having a shape conforming to the shape of said cavity,an inner region consisting of glass transparent to light to which saidphotocathode is sensitive and an outer region consisting of glass whichis opaque to light to which said photocathode is sensitive; and removingsaid block from said mold.
 14. A method as defined in claim 13 whereinthe thickness of said opaque glass disc is less than that of saidtransparent glass disc.